Low loss coaxial conductor using overlapped and insulated helical wound strips

ABSTRACT

A low loss transposed high frequency conductor comprising an overlapped spiral web. The web is continuous, thin and flat and is formed into n layers by overlapping adjacent turns by the fraction (n-1/n). Each layer is insulated from adjacent layers. The conductor can be utilized as the inner conductor of a coaxial cable, the outer conductor of a coaxial cable, a tuning member in a coaxial cavity or as an inductor.

United States Patent [54] LOW LOSS COAXIAL CONDUCTOR USING OVERLAPPED AND INSULATED HELICAL 2,998,840 9/1961 Davis 333/31 C 3,106,768 10/1963 Gent et al. 333/95 X 3,163,836 12/1964 Sugi et a1. 333/96 3,376,378 4/1968 Bullock 174/124 X 3,412,199 11/1968 Yeh 333/96 X FOREIGN PATENTS 950,133 9/1956 Germany 333/95 22,005 8/1906 Great Britain 174/106 OTHER REFERENCES Kimbark, E. W., Electrical Transmission of Power &

WOUND STRIPS H 9 Claims 6 Drawing Figs. Signals, John Wiley & Sons, 1949, pp. 66- 68, 76, 347 350 Primary Examiner-I-Ierman Karl Saalbach [52] US. Cl 333/96, Asst-8mm Examiner wmiam H Punter [51, 1M. .if???iiil lbllfifii H01p7/04,H01b 7/26 [50] Field of Search 333/96, 97; ABSTRACT; A l loss transposed high frequency conductor 174/108, 109, I30, l3l comprising an overlapped spiral web. The web is continuous,

- thin and flat and is formed into n layers by overlapping ad- [56] References cued acent turns b the fraction (n-l/n). Each layer is insulated J 1 UNITED STATES PATENTS from adjacent layers. The conductor can be utilized as the 1,939,451 12/1933 Hull 174/109 inner conductor of a coaxial cable, the outer conductor of a 2,018,477 10/1935 Wentz. 174/124 X coaxial cable, a tuning member in a coaxial cavity or as an in- 2,922,835 1/1960 Lehr 174/36 X ductor.

PATENTED M1824 I911 SHEET 2 [IF 2 LOW LOSS COAXIAL CONDUCTOR USING OVERLAPPED AND INSULATED HELICAL WOUND STRIPS This invention relates to a conductor particularly suited for operation at high frequencies. As the frequency of electrical energy increases, the current which flows through a conductor becomes concentrated near the surface of the conductor. This phenomenon is referred to as skin-effect." As current concentrates near the surface of the conductor, loss of signal energy or attenuation increases.

It has been found that attenuation losses can be reduced if the conductor is segmented and the outer surface transposed with an inner surface at intervals along the conductor length. Conductors have been proposed which are constructed of separate or discrete sections. This is an expensive technique, not practical for mass production. The resulting conductor is heavy and clumsy, not suitable for use in many situations, as where weight and flexibility are important factors. The theory of such conductors is developed in a paper by Raesbeck et al. published in the Bell System Technical Journal, July' 1958, page 835. Examples of prior conductors utilizing this concept are found in Doherty US. Pat. Nos. 2,779,814, 2,8l2,502 and 2,845,473, and McMahon US. Pat. No. 2,872,50l.

In accordance with a preferred embodiment of the invention, the high frequency conductor is a conductive web, wound in a spiral with adjacent turns overlapped and insulated from each other. This construction affords a continuous transposition of the surfaces of the conductor, along its length.

Such a conductor may readily be manufactured by winding a web having an insulating coating on one surface, in a spiral configuration about a cylindrical core.

A major feature of the invention is the provision of a coaxial cable utilizing such a low loss, high frequency conductor. The inner conductor of the coaxial cable is more significant than the outer in establishing the attenuation characteristic of the cable. Accordingly, at least the inner conductor should utilize the low loss construction.

Another feature is that multiple continuous transpositions of the conductor are achieved by overlapping the web more than one-half the web width, as two-thirds or three-fourths the width.

A further feature is that the low loss conductor may be used in circuit elements other than as a part of a coaxial cable. For example, the conductor may be utilized as a low loss inductor; or to form the conducting surfaces of a resonant cavity.

Further features and advantages are readily apparent from the following description of the drawings, in which:

FIG. 1 is a diagrammatic elevation of an overlapped twolayer conductor wound from a web;

FIG. 2 is an enlarged longitudinal cross section of a twolayer conductor utilized as the inner conductor of a coaxial cable, with the layers shown spaced for purposes of illustration;

FIG. 3 is a diagrammatic elevation of an overlapped threelayer conductor wound from a web;

FIG. 4 is an enlarged longitudinal cross section of a coaxial cable utilizing a three-layer conductor as its inner conductor with the layers shown spaced for purposes of illustration;

FIG. 5 is a longitudinal cross section of a. coaxial cavity including the multilayer conductor of FIG. 2 as its tuning member; and

FIG. 6 is an elevation view of an inductor formed from a multilayer conductor.

The invention is illustrated herein asa multilayer conductor embodied in a coaxial cable. The multilayer conductor may be used at frequencies from a few hundred kiloHertz to several hundred megaHertz, and it may be used in coaxial cables having a wide range of impedance. In general, the low loss conductor may be utilized advantageously where aresistance less than that of a solid or tubular conductor is desirable.

Referring now to FIG. I, a continuous web 10 ofa conductor has an insulating layer 11 on itsinner surface and is spirally wound around core 12, to form a tubular conductor. The conductive web l0.may be of copper and the insulating layer may be of polyethylene.

Core 12 is preferably utilized as a base or form on which to wrap the web 10, unless the physical characteristics of the web are such that it can be wound without support. If the spirally wound web is self-sustaining, the core 12 can be removed after the conductor is formed, to reduce weight and increase flexibility. If the core is left inside the conductor, it may be of a conductive material (insulated from the web 10 by insulating layer I l) and form a part of the energy-transmitting structure, or may be an insulator and serve solely as a physical support.

' In FIG. 2, a two-layer conductor 13 (as in FIG. I) is utilized as an inner conductor of a coaxial cable. A cylindrical outer conductor 14 surrounds and is electrically isolated from the inner conductor by physical spacing or by insulation (not shown); Conductor 13 is wound from a conductive web 15 with an insulating coating 16, affixed to its inner surface.

Adjacent turns of the web are overlapped one-half of the width of the web, in FIGS. 1 and 2. To illustrate the overlapping, three successive points of the web 15 will be considered in FIG. 2. The points 17, 18 and I9 depict the position of the web at the start, middle and end of a turn of the spiral. Web 15 is also depicted for illustrative purposes as having three lateral portions, a, b and c. The layers of the conductor are shown separated for clarity. In practice the layers would usually be wound on core 20 (shown in broken lines).

Web portion a at position 17 is connected directly with web portions l8-a and l9-a, forming the outer surface of the conductor. Web portions b (i.e., l7-b, l8-b and 19-b) form an inner surface of the conductor. Web portions l7-c, 18-0 and 19-c provide a transition between the inner and outer layers of the conductor.

Energy travels generally longitudinally along the conductors, the energy being about equally divided between the surfaces formed by the spiral wrapping.

FIG. 3 illustrates a three-layer conductor formed of a web 25 with an insulating inner coating 26 wound on core 27. The formation of three layers is accomplished by overlapping adjacent turns of the continuous conducting web 25 by the fraction two-thirds, leaving a width W/3 exposed. In the general case, a multiple-layer cable having n layers requires that the overlap of adjacent turns be n-l/n. The figure l/n is referred to herein as the overlap factor."

FIG. 4 illustrates a three-layer conductor 29 (as in FIG. 3) forming the inner element of a coaxial cable having an outer conductor 30. Again, the spacing is exaggerated for clarity. The web of conductor 29 has five portions, the portions a, b, and c forming outer, intermediate and inner conductor surfaces with portions e and d providing a transition between surfaces. A consideration of web portions 33a, 32-h and 31-c illustrates the three overlapped conductor surfaces.

Various electrical and physical characteristics determine the dimensional relationships in a conductor embodying the invention.

The thickness of the conductive web is related to the frequency; and should be about 0.85 skin depth at the mean operating frequency. For copper, for example, skin depth At 4 mI-Iz.,

8=1.3 mils and the conductive web should have a thickness of 1.1 mils.

To achieve a high degree of loss reduction, transpositions of the conductive surface should be made every one-eighth wavelength (M8) and more frequent transpositions are desirable. At 4 mHz., a wavelength is about 250 feet and this is not a critical limitation. However, at higher frequencies it becomes more important. The width of the conductive web (W) can be selected for ready availability and ease in handling, for all except very high frequency conductors, where the spacing of transpositions'becomes small.

The thicknessof the insulation layer (11, 26) on the inner surface of the conductive web should be as small as possible. A V4-mil plastic filmmight be used, for example.

For a conductive webof width W, wound to form a conductor of radius a with an overlap factor l /n, the angle of wrap, the angle I), between the edge of the web and a plane at right angles to the longitudinal axis of the conductor (FIG. 1) is determined by sin q --W/2mra. This relation requires that W 3 21mm.

The invention can be hEli zeddh high frequency devices other than coaxial cables. Examples are illustrated in FIGS. 6 and 7.

A resonant coaxial cavity, FIG. 5, has conductive walls which define a chamber of cylindrical cross section. Input and output coupling loops 51 and 52 are diametrically opposed on the cylindrical surface of the chamber. An inner conductor 53 utilizes the transposed conductive surfaces of this invention and is provided with an adjustable shorting member 54 to tune the resonant frequency of the chamber. The 'lowlosses of the multilayer conductor .53 reduce the energy loss within the cavity, providing a significant increase in the cavity Q. If a further increase in Q is desired, the walls 50 of the cavity may also be comprised of a multilayer conductor.

The multilayer conductor may also be utilized by itself, as for example in the simple, helically wound inductor 60, FIG. 6. This inductor, because of the decreased resistance, will exhibit a higher Q than an inductor formed from a tubular or solid conductor.

The low loss high frequency conductor of this invention is inexpensive and simple to construct and can be commercially mass produced. It provides a practical embodiment for the mechanically complex conductors of the prior art.

Iclaim:

1. In a coaxial cable having inner and outer conductors, the improvement comprising at least one of said conductors including a continuous, conductive foil strip spirally wound upon itself with adjacent turns partially overlapped, said foil having a thickness of less than about the skin depth for the mean average frequency of operation for said cable, and insulating means for insulating overlapped portions of said foil whereby induced current will flow in a plurality of overlapped portions of said foil thereby to reduce the overall skin effect resistance of said conductor.

2. The cable of claim 1 wherein said one conductor is the length of each of said transposed conductive surfaces is less than about one-eighth wavelength of the mean operating I frequency of said cable.

6. The cable of claim 1 wherein the foil ofsaid conductor has a width W and forms a cylindrical, tubular conductor, of radius a, including n layers, and wherein the spiral has a pitch angle 1 suchthat 7. The cable of claim 1 wherein the web forms n superposed layers and the fraction of overlap of adjacent turn is (nl )/n.

8. A coaxial cavity formed by a short circuited section of a coaxial transmission line wherein the transmission line includes:

a tubular conductive tuning member of an overlapped, spiral web wherein layers are formed by the spiral web with adjacent turns overlapped, said layers being insulated from each other;

a conductor, for short circuiting the conductive tuning member; and an outer conductor, surrounding the mner conductor and having two apertures therein for coupling energy into and out of the cavity. 9. The coaxial cavity of claim 8 wherein the outer conductor defines a cylindrical cavity and the apertures are diametrically opposed on the cylindrical surface. 

1. In a coaxial cable having inner and outer conductors, the improvement comprising at least one of said conductors including a continuous, conductive foil strip spirally wound upon itself with adjacent turns partially overlapped, said foil having a thickness of less than about the skin depth for the mean average frequency of operation for said cable, and insulating means for insulating overlapped portions of said foil whereby induced current Will flow in a plurality of overlapped portions of said foil thereby to reduce the overall skin effect resistance of said conductor.
 2. The cable of claim 1 wherein said one conductor is the inner conductor and further comprising an inner core upon which said foil is wound.
 3. The coaxial cable of claim 1 wherein said insulating means comprises a layer of polyethylene affixed to one side of said foil.
 4. The cable of claim 1 wherein the thickness of said foil is about 0.85 skin depth at the mean operating frequency.
 5. The cable of claim 1 wherein each spiral winding of said foil produces a plurality of transposed conductive surfaces, and wherein said conductor is characterized in that the axial length of each of said transposed conductive surfaces is less than about one-eighth wavelength of the mean operating frequency of said cable.
 6. The cable of claim 1 wherein the foil of said conductor has a width W and forms a cylindrical, tubular conductor of radius a, including n layers, and wherein the spiral has a pitch angle phi such that sin phi W/2n pi a.
 7. The cable of claim 1 wherein the web forms n superposed layers and the fraction of overlap of adjacent turn is (n-1)/n.
 8. A coaxial cavity formed by a short circuited section of a coaxial transmission line wherein the transmission line includes: a tubular conductive tuning member of an overlapped, spiral web wherein layers are formed by the spiral web with adjacent turns overlapped, said layers being insulated from each other; a conductor, for short circuiting the conductive tuning member; and an outer conductor, surrounding the inner conductor and having two apertures therein for coupling energy into and out of the cavity.
 9. The coaxial cavity of claim 8 wherein the outer conductor defines a cylindrical cavity and the apertures are diametrically opposed on the cylindrical surface. 